The present invention relates to an improved apparatus for randomly dispersing a fluid blowing agent throughout a thermoplastic material of high viscosity in the molding of foamed thermoplastic articles characterized by a cellular core and a substantially non-cellular, integral peripheral shell.
In order to obtain a foamed thermoplastic article, it is necessary that a fluid blowing agent be mixed with the foamable thermoplastic material. In the conventional operation of molding foamed thermoplastic articles, as described in U.S. Pats. Nos. 3,268,636 and 3,436,446 the fluid blowing agent is introduced into an extruder containing foamable thermoplastic material and maintained at a temperature above the melting temperature of the thermoplastic and at a pressure above the foaming pressure of the fluid blowing agent. The mixture is passed into an accumulation zone and thereafter foamed and molded in a mold. The mixing of the fluid blowing agent and the foamable thermoplastic occurs only as the materials are passed through the extruder.
It is desirable that foamed thermoplastic articles characterized by a cellular core and a substantially non-cellular peripheral shell have uniform cell structure of controllable size. Such products require uniform dispersion of the fluid blowing agent throughout the foamable thermoplastic materials. However, the incompatibility of the fluid blowing agent and the thermoplastic material makes uniform dispersion in an extruder extremely difficult.
An improved extrusion apparatus is disclosed in U.S. Pat. No. 3,329,882. This invention provides in combination with an extruder an apparatus adapted to provide high and low shearing force. This apparatus has been found desirable when used to mix solids, as when color blends are used and uniform color dispersion is desired. Also, when working with high density or linear polyethylene which is to be extruded in the form of thin films, this apparatus prevents gel particles or fish eyes (particles which have not melted or been plasticized) from being extruded. However, this apparatus has not been suitable for mixing low viscosity fluid blowing agents with high viscosity thermoplastic melts such as in the molding of foamed thermoplastic wherein a low viscosity blowing agent has to be dispersed throughout a high viscosity thermoplastic material. This apparatus does not provide a means for breaking up the low viscosity fluid and it has been found that simply subjecting a mixture to shearing force without first breaking up the low viscosity fluid is inadequate for obtaining thermoplastic articles characterized by uniform cell structure of controllable size.
This invention providea apparatus for mixing a low viscosity fluid blowing agent with a high viscosity thermoplastic melt which is relatively simple in construction for accomplishing the same.